In a typical multiple-access communication system such as, for example, packet switching systems, access to the network is by demand. That is, each user seeks to use the network as the need arises, and the transmission is broadcast to all the other users. Address information is contained in a header as part of the transmitted information and the receiving stations listen to the transmissions from all participating members of the network. Each selects that information which is directed to it.
The most common method of allocating resources in packet switching system is by contention, which gives rise to a serious problem when two or more stations attempt to transmit simultaneously. (See U.S. Pat. No. 4,063,220). When this occurs, the transmissions from all the contending stations are garbled and none is properly received by the addressees. Clearly, some means must be provided so that each subscriber can ascertain when this has occurred and, thereby, know when the lost data must be retransmitted. The most common method is for the transmitting station to listen as it transmits, and to be sure it receives a correct replica of what was transmitted. However, in order for this test to have validity, the minimum packet duration must be at least as long as the total transmit time across the network or else virtual destruction of transmissions could occur at one point in the network and be undetected at another. At low bit rates and over small distances, this minimum packet size restriction is of little consequence. However, as bit rates increase, and as network dimensions increase, this constraint may well impose a minimum packet size which is larger than the customer needs.
Another disadvantage of packet-switch systems is that it is inherently inefficient in its use of facilities. In a simple contention system where the packet duration is shorter than the propagation time, the maximum throughput is about 18 percent of the nominal system capacity. In more sophisticated systems, this efficiency may increase to about 36 percent. A further disadvantage, and perhaps the most serious of all is that on overload, packet-switched contention systems degrade most ungracefully. As the network gets more and more crowded, the number of improperly received transmissions increases and, therefore, the number of retransmissions increases. This only serves to add additional load to the already overloaded system. The result is often devastating.
To avoid these limitations various techniques are employed whereby the network is made available to the users in accordance with some sort of plan. For example, in U.S. Pat. No. 4,199,661 the nontransmitting stations are locked out when the channel is active. To avoid two or more users seizing the channel when it becomes idle, each station is inhibited from transmitting for some unique, predetermined period of time. That is, a priority system is imposed upon the users. While this may work well for high priority users, it can, in effect, prevent lower priority users from getting access to the network during busy periods.